Perceived roughness is a common problem for some diesel engines when operating at idle conditions and light loads. Variation of fuel delivery to the different cylinders is one of the major sources of idle roughness. At the engine manufacturing facility it is desirable to conduct diagnostic tests to determine if the idle roughness is acceptable before the engine is shipped.
It has previously been known to provide diagnosis of idle roughness and quality at the engine manufacturing location. One common method, listening to the engine idle in a test cell, is not conclusive as to the level of idle quality. If the idle quality is checked after it is installed in the end vehicle application and found unacceptable, it can be very expensive to make the necessary changes to the fuel system. Some fuel systems do not have the capability to adjust the balance of fuel to the various cylinders at both no load and full load. Fuel injectors are typically calibrated at idle fuel on a test bench before installation in the engine. However, there is variability in the calibration. In addition, the injectors need to be indexed uniformly when installed in the engine and there is further variability in this procedure.
Generally, if the injectors are calibrated properly and if the indexing of the injectors in the engine is accurate, there is no problem with idle roughness. However, if these processes are not controlled, an unsatisfactory level of idle roughness can be present. Thus a method is needed to diagnose the idle roughness quality while the engine is still in the engine manufacturing facility so that problems can be addressed before the engine leaves the facility. A previous method used was a Fourier transform of the engine speed signal. This method worked for cases where the idle roughness was far out of specification. However, it was not sensitive enough to detect the vast majority of idle roughness problems.